[clang] [clang] Implement __builtin_popcountg (PR #82359)

[Nick Desaulniers via cfe-commits]

nickdesaulniers wrote:

Looks like the linter is upset with pre-existing changes.  ok2land. Are you able to merge this @overmighty or do you need us to?

https://github.com/llvm/llvm-project/pull/82359